Modem computer systems have driven a demand for enormous amounts of data storage. Data traditionally has been stored in one or more mass data storage devices, such as rotating magnetic disk drives or tape drives, attached to a single computer system. As computer systems have become larger, faster, and more reliable, there has been a corresponding increase in need for storage capacity, speed and reliability of the storage devices. Increases in the data storage capacity and reliability of storage devices have been dramatic in recent years. But with all the improvements to the devices themselves, there are certain limitations to what can be accomplished. Additional configurations of storage devices have increasingly been offered in recent years to meet demand for larger capacity, faster, more reliable, and more accessible data storage.
One example of alternative configurations is the rapidly increasing popularity of so-called "RAIDs", i.e., redundant arrays of independent disks. A RAID stores data on multiple storage devices in a redundant fashion, such that data can be recovered in the event of failure of any one of the storage devices in the redundant array. RAIDs are usually constructed with rotating magnetic hard disk drive storage devices, but may be constructed with other types of storage devices, such as optical disk drives, magnetic tape drives, floppy disk drives, etc. Various types of RAIDs providing different forms of redundancy are described in a paper entitled "A Case for Redundant Arrays of Inexpensive Disks (RAID)", by Patterson, Gibson and Katz, presented at the ACM SIGMOD Conference, June, 1988. Patterson, et al., classify five types of RAIDs designated levels 1 through 5. The Patterson nomenclature has become standard in the industry.
Another example of a storage alternative is the concept of a storage subsystem. A storage subsystem implies a greater degree of independence from a host computer system than is typically found in an isolated storage device. For example, the subsystem may be packaged in a separate cabinet, with its own power supply, control software, diagnostics, etc. The subsystem may have a single storage device, but more typically contains multiple storage devices. The notion of a storage subsystem and a RAID are not necessarily mutually exclusive; in fact, many RAIDs are constructed as semi-independent storage subsystems, which communicate with a host through a communication link having a defined protocol. It is possible in such subsystems that the host is not even aware of the existence of multiple data storage units or data redundancy in the storage subsystem. To the host, the subsystem may appear to be a single very large storage device.
A configuration of multiple storage devices need not be attached to only a single host computer system. It might be that multiple computer systems are configured to share multiple storage devices. Thus, configurations of storage devices can be generalized to the concept of a storage network.
As used herein, a storage network is a configuration of multiple data storage devices, connected to one or more host computer systems, such that there is a communication path from each storage device to each host system which does not cross the system bus of another host system. Because there exists a direct communication path from each system to each storage device, data on any device is readily accessible to any of the systems. Storage devices in a storage network are not necessarily identified with or controlled by a host. This latter feature distinguishes a storage network from a simple network of computer systems, each having its own local storage devices. Thus, in certain computing environments, a storage network facilitates sharing of data and improved performance over a conventional network of host systems.
While it is theoretically possible to construct complex storage networks shared among multiple host computer systems using prior art hardware, in reality this is an error-prone and difficult task. Documentation and software support may be primitive or non-existent. A user must determine how to operate, configure and attach devices. Optimum physical configurations may depend on logical configurations and modes of operation, such as one or more RAID levels. There may be numerous hardware dependencies and limitations, such as number and type of devices that may communicate with a single I/O controller or adapter. All these requirements place substantial demands on the expertise of the user. It would be desirable to support the construction of storage networks with software which assists the user.